1. Technical Field
The present disclosure relates to a semiconductor light emitting device, and in particular to a semiconductor light emitting device that includes a quantum well active layer.
2. Description of Related Art
Semiconductor light emitting devices such as light emitting diode (LED) and laser diode (LD) are typically and widely used for various light sources for backlighting, lighting fixtures, traffic lights, large-size displays, and the like. In particular, a light emitting device that includes gallium nitride group (GaN group) compound semiconductor is used as short wavelength light emitting device.
This type of semiconductor light emitting device can include an n-type semiconductor layer, which is deposited on or above a substrate, and a p-type semiconductor layer, which is deposited after the n-type semiconductor layer is deposited. In addition, an active layer is interposed between the n-type and p-type semiconductor layers. In order to improve light emission efficiency, the active layer has a quantum well structure in which a plurality of well layers and a plurality of barrier layers are alternately deposited on one another. For example, in the case where the well and barrier layers are formed of InGaN and GaN, respectively, piezoelectric polarization is produced by the lattice constant difference at the boundary between the well and barrier layers. It is known that the piezoelectric polarization produces the piezoelectric field, which causes distortion of the band gap.
FIG. 8 shows the profile of composition ratio of In along the deposition direction of InGaN well layers 20 and GaN barrier layers 22 where the concentration of In is fixed. FIG. 9 shows the band profile of the active layer having this profile of composition ratio of In where the piezoelectric field does not produce the distortion of the band gaps, while FIG. 10 shows the band profile of the active layer where the piezoelectric field produces the distortion of the band gaps. In the case where the piezoelectric field produces the distortion of the band gaps, the positive polarization charge will be produced on the side opposite to the substrate side of each of the well layers 20, while the negative polarization charge will be produced on the substrate side of each of the well layers 20.
Due to the polarization charge, the energy of the conduction band and the valence band of each of the well layers 20 becomes higher on the substrate side, and becomes lower on the side opposite to the substrate. The electrons collect on the side opposite to substrate side of each of the well layers 20, while the positive holes collect on the substrate side of each of the well layers 20. Since the electrons and the positive holes are located apart from each other, the wave function of electrons shifts to one side, while the wave function of positive holes shifts to another side. Accordingly, the recombination probability will be reduced between the electrons and the positive holes.
To address this, it can be conceived that the well layer is constructed to have a gradient composition ratio of In along the deposition thickness direction. JP 2003-60232 A and 2005-56973 A disclose that the concentration of In is linearly reduced from the substrate side to the side opposite to the substrate. FIG. 11 shows the profile of composition ratio of In (lower side) and the band profile (upper side) in the linearly reduced concentration of In.
(See also JP H11-026812 A and JP 2008-288397 A.)
In this case, the electrons will be uniformly located in the conduction band 23 of the well layer 20 that is flat in the thickness direction. However, the positive holes will collect around the peak 24 of the valence band on the substrate side. For this reason, the probability of Auger recombination will be increased in which no light is emitted. Also, the recombination probability of the electrons and the positive holes will be reduced. In addition, in the case where the positive holes collect around the peak 24 of the valence band of each of the well layers 20, the tunneling distance D is increased in which the positive holes are required to move between the well layers 20 adjacent to each other through the tunnel effect. Accordingly, there is a problem that the carrier mobility is reduced.
The present invention is aimed at solving the problem. It is an object of the present invention to provide a semiconductor light emitting device having improves light emission efficiency.